Most women ovulate about halfway into their menstrual cycles (if you count the first day of your period as day one). For a very rough estimate, subtract 14 to 16 days from the date you expect your next period to begin to get an idea of when you may ovulate.
• So if you know your period's due on the 30th of the month, you'll probably ovulate between the 14th and the 16th.
• If your period's due on the 18th of the month, you probably ovulate between the 2nd and the 4th.
However, when you ovulate depends on the length and regularity of your menstrual cycle -- and it can be harder to tell when you're ovulating if your cycle varies from month to month. Some physical symptoms may accompany ovulation, but they're subtle enough that not everyone can spot them. About 20 percent of women experience lower abdominal pain (it's called mittelschmerz) when ovulation occurs. Breast tenderness is also common.
You can try tracking your basal body temperature (your body's temperature first thing in the morning) as a way to figure out when you're ovulating. BBT increases slightly -- about half a degree -- when ovulation occurs. (Note: Because this spike is so small, you need a special basal body thermometer to measure it). Your vaginal discharge can provide a clue as well: Most women have a clear and somewhat elastic (try stretching it between two fingers) discharge in the days leading up to ovulation.
The most accurate way to know when you're ovulating is to use an ovulation predictor kit (OPK). These work by detecting a surge in luteinizing hormone (LH), which your body releases up to two days before you ovulate. Because these kits give you the most advance warning (compared to checking basal body temperature or watching for physical signs), they're your best bet for knowing when to time sex to coincide with ovulation.